Multi-ply web forming method and apparatus and a multi-ply paper or board product formed hereby

ABSTRACT

A multi-ply web forming method and apparatus are disclosed for forming a top ply onto a base ply. A fiber suspension jet is by means of a secondary headbox ( 21; 21′, 26 ) delivered into a twin-wire roll nip created by two tensioned wires ( 16, 22; 16′, 22′; 22, 27 ) one of which ( 16; 16′, 22 ) carries the moist base ply. The web forming of the top ply is performed solely by means of roll forming ( 23, 23′, 28 ) of the kind where the fiber suspension jet is delivered to said twin-wire nip at such a high speed to cause a yielding deflection of the outer of said two tensioned wires, while maintaining substantially constant tension during said deflection of the outer wire by guiding said wire on rotating supports ( 31   a-c   ; 31   a′-c′   ; 32   a-c ) at least one of which is resiliently or displaceably mounted to compensate for said deflection, wherein the speed of said fiber suspension yet delivered to said twin-wire nip is at least 300 m/min and the wire tension of the outer as well as the inner wire is at least 4 kN/m.

This invention concerns a method for high speed forming of multi-plypaper or board, a forming apparatus utilizing the method and a paper orboard product formed by the method and/or the apparatus according to theinvention.

BACKGROUND OF THE INVENTION

Multi-ply forming enables the cost performance relationship of theproduct to be optimized by using different furnishes in different plies.For many paper products multi-ply technology makes it possible toincrease the content of recycled paper and high yield pulps which areinteresting both for cost reducing reasons and for environmentalreasons. Existing technology fails however to accomplish the task ofhigh speed forming of multi-ply paper or board with superior mechanicalproperties as well as favourable ply coverage characteristics.

The need for a forming technology applicable to high production rates isstressed by recent developments in wet pressing technology. Shoe pressesgiving high press impulses and high pressing efficiency are now beinginstalled in the production of most major paper and board grades.

Good ply coverage characteristics, i.e. good formation and purity of theindividual plies, is an obvious requirement in order to fully utilizethe potential of a multi-ply product. The need for a forming technologyyielding superior mechanical properties is stressed by the growinginterest to use raw materials with a relatively low strength potentialsuch as recycled fibres and high yield pulps.

Multi-ply forming technology may be grouped into three main categories:

1. Forming each ply in a separate forming unit before couching the pliestogether.

2. Simultaneous forming of all plies in one forming unit using amulti-layer headbox.

3. Forming the web plies on top of each other in a sequential mode, i.e.forming the second ply on top of the first ply and the third ply on topof the second ply etc. The present invention belongs to this category.

Separate forming is commonly carried out with a multi-fourdriniermachine. Hybrid forming or twin-wire forming (cf. e.g. DE 44 02 273 C2)may also be applied. The increase in dewatering capacity given by two ormore separate forming units may be utilized for increased productionrates and/or lowered forming consistency for improved sheet properties.All variants of separate forming have one problem in common, however,viz. the ply bonding which generally limits the Z-direction strength ofthe multi-ply product. Often starch or some other bonding agent has tobe sprayed on the plies before couching them together.

While twin-wire forming would be preferable for speeds above 1000 m/min,avoiding free surface instabilities and providing higher dewateringcapacity the problem of ply-bonding then becomes worse. This is becausea twin-wire-formed sheet ply has two wire sides with poor ply-bondingability, in contrast to a fourdrinier ply which has one wire side andone top side with a better ply-bonding ability.

Simultaneous forming of a multi-ply product with a multi-layer headboxmay also be employed. Examples of multi-layer headboxes are found in EP0 681 057 A2 and in GB 2 019 465. With this method, however, thedewatering capacity is limited to that given by a single dewateringunit. Hence, this principle is not suitable for high speed forming, ofmoderate to high grammages at low forming consistency. Hitherto, it hasmoreover proved difficult to accomplish acceptable ply coveragecharacteristics with simultaneous forming.

Multi-ply forming in a sequential mode has traditionally been applied inthe forming of two-ply linerboard using a secondary headbox placed somedistance downstream a fourdrinier wire with dewatering of the top plythrough the base ply formed upstream of the secondary headbox. Theproblem of ply-bonding is essentially avoided by depositing a fibresuspension onto the pre-formed web. This means that the Z-directionstrength of the multi-ply product is often determined by the Z-directionstrength of the individual plies rather than by the ply-bonding. Forminga top ply onto a base ply on a fourdrinier wire involves severaldisadvantages, however. It suffices to mention the drawback regardingdewatering capacity and the severe grammage variations due to the freesurface occurring especially above 1000 m/min.

Dewatering in a twin-wire zone created by the wire carrying the base plyand an additional, web-free wire through which the top ply isessentially dewatered has been applied shortly after the secondaryheadbox in units relying extensively on vacuum generated dewatering (seee.g. Attwood (1991) “Multi-ply forming”, Pulp and Paper manufacture Vol.7 Paper Machine Operations, TAPPI & CPPA; p.250-251). Dewatering throughthe previously formed web is thus essentially avoided whereby animproved dewatering efficiency is achieved. Dewatering of the top plythrough a web-free wire is moreover advantageous with regard to purityand formation of the top ply, because a separate handling of thewhite-water of the top ply is possible and because any influence of thebase ply structure on the top ply is substantially avoided. The capacityof this kind of units is still limited, however, and they are typicallyused on multi-ply board machines running slower than 600 m/min.

In U.S. Pat. No. 3,543,834 there is disclosed a multi-ply web formerutilizing cylinders or rolls. Successive web plies are formed in aforming area between the foraminous belts wrapping a forming cylinderwhere one of the belts are used in the preceding forming area. Accordingto U.S. Pat. No. 3,543,834 dewatering is accomplished by “centrifugalforce and pressure of the foraminous belt against the web”.

There is no indication of conditions involving the impinging headbox jetto deflect the outer wire and penetrate into the twin-wire nip. Rather,it can therefore be inferred that the wire geometry is fixed. This meansthat the principal forming phase is not accomplished over the rollperiphery at an essentially constant dewatering pressure. It is then notpossible with this arrangement to achieve favourable mechanical sheetproperties, since a substantially constant dewatering pressure is aprerequisite for good mechanical properties. The dewatering capacity ismoreover unsatisfactory with this arrangement.

U.S. Pat. No. 3,625,814 discloses a multi-ply web former of a similarkind. Dewatering of the pulp stock is said to take place “as the beltscome together on the impervious forming roll”, which indicates that thegeometry of the outer wire is fixed.

The same applies to the multi-ply web former disclosed in U.S. Pat. No.3,821,073. The fibre suspension is dewatered “in that the water isforced through the two wires as these run together along a portion ofthe cylindrical surface of the forming roll”.

In DE 44 02 273 A1 there is disclosed a two-ply forming unit utilizingtwin-wire roll-blade forming for both the base ply and the top plyformed onto the base ply. Roll-blade forming employs only an initialroll dewatering phase followed by blade dewatering, During roll forming,which was introduced in its basic sense some four decades ago (U.S. Pat.No. 3,056,719) and is well known from the field of high speed productionof (single-ply) printing paper, the two wires containing the fibresuspension runs on the periphery of the rotating forming roll. Thedewatering pressure is determined by the outer wire tension divided bythe instantaneous radius of curvature, and during roll dewatering thepressure rises steeply during an initial phase after which it levels offto a plateau. During blade dewatering the wires are deflected overstationary blades resulting in a pulsating dewatering pressure.

Although the employment of roll-blade dewatering means a significantlybetter machine speed potential than the previously described methods forforming a top ply onto a base ply, it has still drawbacks, in particularwith regard to the mechanical sheet properties. The blade dewatering canhave a strong adverse effect on the Z-direction strength of theindividual plies, meaning that the Z-direction strength of the multi-plyproduct remains to be a problem despite that the top ply is formed ontothe base ply, Moreover, the blade dewatering tends to deteriorate themechanical properties in the plane.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method for forming atop ply onto a base ply utilizing a twin-wire unit, in which theshort-comings of existing technology are avoided. This object hasaccording to the invention been achieved by carrying out the web formingof the top ply after said twin-wire roll nip solely by means of rollforming of the kind where the fibre suspension jet is delivered to saidtwin-wire nip at such a high speed to cause a yielding deflection of theouter of said two tensioned wires, while maintaining substantiallyconstant tension during said deflection of the outer wire by guidingsaid wire on rotating supports at least one of which is resiliently ordisplaceably mounted to compensate for said deflection, wherein thespeed of said fibre suspension yet delivered to said twin-wire nip is atleast 300 m/min and the wire tension of the outer as well as the innerwire is at least 4 kN/m.

With base ply is meant a previously formed ply onto which a further ply,the top ply, is formed. The base ply may consist of more than one plyand by repeated usage of the method according to the invention amulti-ply product with an arbitrary number of plies can be formed.

In multi-ply forming, certain advantages may be achieved by forming one,or a few of the plies according to the invented method, e.g. web pliescontaining weak pulps (high yield pulps or recycled fibres). The task ofhigh speed forming of a multi-ply web with superior mechanicalproperties and good ply coverage characteristics is however bestaccomplished by the employment of roll forming of the above stated kindfor all plies, including the primary ply.

The invention also refers to a forming apparatus for performing themethod comprising a secondary headbox arranged to deliver a fibresuspension jet into a twin-wire roll nip created by two tensioned wiresone of which carries the moist base ply, said apparatus comprises as thesole forming unit for forming of the top ply onto the base ply, a rollforming unit including at least one forming roll of the kind where thefibre suspension jet is delivered to said twin-wire nip at such a highspeed to cause a yielding deflection of the outer of said two tensionedwires, said apparatus further comprising means for maintaining asubstantially constant tension during said deflection of the outer wireby guiding said wire on rotating supports at least one of which isresiliently or displaceably mounted to compensate for said deflection.

The invention further refers to a multi-ply paper or board productformed by the method and/or apparatus defined above.

Further details and characteristics of the invention are stated in thefollowing description and in the dependant claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic principle for forming a top ply onto a base ply.

FIGS. 2 and 3 show two examples of forming sections for a two-ply weband a three-ply web respectively, both of which employ roll forming forthe primary web ply and the method according to the invention forforming the other web plies.

DESCRIPTION OF EMBODIMENTS

The invented method for forming a top ply onto a base ply isschematically shown in FIG. 1. A fibre suspension jet 10 exiting from aheadbox 11 enters a nip created by one tensioned forming wire 12 caringa moist web, preferably of 7-15% dryness, and another web-free,tensioned wire 13 both of which wrap around a rotating forming roll 14.The ingoing moist web is preferably carried by an inner wire as shown inthe figure and the impingement of the headbox jet is preferably inclinedtowards the outer wire in order not to deteriorate the ingoing moistweb. The headbox jet is delivered to said twin-wire nip at such a highspeed to cause a yielding deflection of the outer 13 of said twotensioned wires as disclosed in U.S. Pat. No. 3,056,719, the content ofwhich is included as a reference in the present application. The tensionof the outer wire 13 is maintained substantially constant during thedeflection by guiding said wire 13 on one or more rotating supports 30 a(only one shown in FIG. 1) at least one of which is resiliently ordisplaceably mounted to compensate for said deflection. The forming roll14 may have a solid or open surface, supported with vacuum or not. Inorder to reach a sufficient dewatering capacity and still to limit thedewatering pressure and thereby the jet deceleration in the twin-wirenip, the forming roll radius is at least 600 mm, preferably at least 200mm. For a sufficient dewatering capacity, the roll wrapping angle of theouter wire 13 preferably larger than 100 degrees. For higher capacitystill, two or more forming rolls may be used as exemplified in thefollowing embodiments.

The speed of said fibre suspension jet delivered to said twin-wire nipis at least 300 m/min in order to create a sufficient high speed andkinetic energy of the fibre suspension jet to cause the yieldingdeflection of the outer wire 13. In some cases speeds of at least 500m/min or at least 800 m/min are preferred.

The thickness of the fibre suspension jet delivered to said twin-wirenip is preferably restricted to 15-20 mm in order to limit the outflowof fibre suspension at the edges of the machine. This together with alow headbox consistency, preferably below 0.5% for typical furnishes, inorder to reach the required sheet properties means that the ply grammageof a top ply which according th the invention is formed onto a base plyis limited to 90 g/m², preferably to 70 g/m².

The wire tension of the outer as well as the inner wire is at least 4kN/m, and the wire tension of at last the outer wire is preferably atleast 6 and most preferably at least 8 kN/m in order to obtainsufficient stability, especially at high speeds.

According to the invention, the forming phase is completed during rolldewatering whether occurring on one or more forming rolls. Thereafterthe fibre network structure is essentially fixed so that any significantrearrangement of the fibres does not occur as the web passe over furtherdewatering elements. Further consolidation of the web may then beaccomplished according to well-known methods such as couch rolls,suction boxes or the like before the web enters the press section oranother forming unit.

The present invention gives the following particular advantages:

A substantially constant dewatering pressure can be obtained as thefibre suspension jet deflects the tensioned outer wire with dewateringtaking place over the periphery of the forming roll.

The dewatering capacity per unit drainage area of a roll forming unit ofa type here employed is high which enables low forming consistencies(0.5% by weight and lower). A high dewatering capacity is especiallyimportant in the multi-ply forming method here presented, in which a topply is formed onto a base ply and dewatering takes place only throughthe outer wire.

An impingement of the fibre suspension jet exiting from the top plyheadbox into a rwin-wire roll nip which means a general insensitivity tojet impingement conditions.

A closed forming zone which is a prerequisite for small grammagevariations at machine speeds above 1000 m/min.

Machine speeds higher than 1000 m/min can be operated in a compactforming section with good runability, including high wire retention,according to the well known performance of roll formers.

A good ply-bond, achieved because the top ply comes in contact with thebase ply while being in the form of a fibre suspension.

A broad range for the degree of fibre orientation, similar to that offourdrinier machines, is possible since orientation effects duringdewatering can be essentially avoided.

Favourable mechanical properties of the individual plies can be obtainedbecause a low forming consistency is possible with two or more formingunits and because detrimental shear can be avoided during the rolldewatering.

A good purity and formation of the web ply can be obtained, dewatering alow consistency fibre suspension through a web-free wire.

In order to realize the potential for mechanical properties in generaland the Z-direction strength in particular, the shear between the fibresuspension and the wires during roll dewatering should be minimal.Therefore, the required degree of fibre orientation should preferably begenerated already in the headbox rather than by means of a speeddifference between the fibre suspension and the wires during dewatering.The speed difference between the fibre suspension and the wires duringthe dewatering can then be restricted to maximum of ±40 m/min relativelythe point of minimum shear.

The degree of fibre orientation in the jet exiting from a hydraulicheadbox is governed by the relative influence of turbulence andelongational strain. Turbulence is generated in the tube bank and has arandomizing effect, whereas elongational strain is imposed in theconverging nozzle and has an orientating effect. For a demonstration ofthe influence of headbox variables on the fibre orientation degreereference is made to papers by Nordström and Norman (Nord. Pulp Pap.Res. 9(1): 53 (1994); 10(1): 33 (1995); J. Pulp Pap. Sci. 21(7): I223(1995)). As an example, it has been demonstrated that with a rube bankdesign involving a high open nozzle feed area and a high nozzlecontraction ratio, a fibre orientation degree corresponding to a tensilestiffness MD/CD-ratio above four can be reached at the point of minimumshear during roll dewatering.

FIG. 2 schematically shows a forming section employing roll forming forthe primary web ply and the method according to the invention for thesecondary web ply. The figure shows a design for a rwo-ply web, but itis understood that the design principles can be applied for a productcomprising more plies. The primary ply is formed in a first twin-wireunit comprising a primary headbox 15 which delivers a fibre suspensionjet into a nip created by a first endless, tensioned forming wire 16 anda second endless, tensioned forming wire 17. Both forming wires 16, 17travel on rotating rolls 18,19 in a S-wrap. The forming phase may thenend either on roll 18 or on the vacuum supplied roll 19 on which furtherdewatering thereafter occurs.

The second wire 17 is then separated from the web over a suction box 20,which secures the web to the first wire 16, which transfers the web to asecond twin-wire unit. Here a secondary web ply is formed on the moistprimary web ply, which preferably has a dryness of 7-15% by weight. Thissecondary unit comprises a secondary headbox 21 which delivers a fibresuspension jet into a twin wire nip created by the first wire 16 and athird endless, tensioned wire 22. The fibre suspension jet is deliveredto said twin-wire nip at such a high speed to cause a yieldingdeflection of the outer 22 of said two tensioned wires as disclosedabove with respect to FIG 1. The outer wire 22 is guided on rotatingsupports 31 a-c at least one of which is resiliently or displaceablymounted to compensate for said deflection. Both wires 16, 22 travel anrotating rolls 23 and 24 in a S-wrap where the secondary web ply isformed onto the primary web ply. The forming phase may then end eitheron roll 23 or on the vacuum supplied roll 24 on which further dewateringthereafter occurs.

The third wire 22 is in the same way as in the first forming unitseparated from the web over a suction box 25, which secures the two-plyweb to the first wire 16, which transfers the sheet to the press section(not shown).

In order to minimize any detrimental effect on the sheet properties dueto velocity differences between the two wires running in S-wrap, a largediameter is recommended for both rolls, preferably in the range1200-1600 mm.

The configuration shown in FIG. 2 gives a particular advantage withregard to the tension of the outer wires, the second and third wiresrespectively 17 and 22, on the forming rolls determining the dewateringpressure during roll dewatering. Since the wire is in direct contactonly with rotating machine elements giving a minimum of wire wear, thewire tension and thus the dewatering pressure can be kept at a higherlevel than if the wire runs over stationary elements.

FIG. 3 shows another design employing roll forming for the primary webply and the method according to the invention for the secondary andthird web plies. The figure shows a configuration for a three-ply webbut even this configuration can of course be applied to the forming of aweb comprising any number of plies. A third ply is formed onto thesecondary web ply by a third twin-wire unit comprising a third head box26 and a twin-wire nip created by the third tensioned wire 22 and afourth tensioned wire 27. The fibre suspension jet is delivered to saidtwin-wire nip at such a high speed to cause a yielding deflection of theouter 27 of said two tensioned wires as disclosed above. The outer wire27 is guided on rotating supports 32 a-c at least one of which isresiliently or displaceably mounted to compensate for said deflection.Both forming wires travel on rotating rolls 28,29 in a S-wrap. Theforming phase may then end either on roll 28 or on the vacuum suppliedroll 29 on which further dewatering thereafter occurs.

In contrast to the forming section shown in FIG. 2 this forming sectioncomprises a sequence of tensioned, endless forming wires, each of whichtransfers the web from one forming unit to the next or to the presssection. An advantage with this arrangement is the compact design. Asclear from the figure, this design involves web transport on theunderside of a single wire in some sections. This is however possibleprovided the web secured to the conveying wire by means of vacuum on thecouch roll.

The invention is of course not limited to the embodiments shown anddescribed above, but several modifications thereof are possible withinthe scope of the claims. The invention also contemplates that any of theheadboxes used may be a multi-layer headbox.

What is claimed is:
 1. A method of forming a multi-ply web of paper orboard, comprising the steps of: carrying a moist base ply on one of twotensioned members of a twin wire nip; delivering a fibre suspension jetby means of a secondary headbox into the twin-wire roll nip to create atop ply on the base ply; and forming the top ply by roll forming a rollforming unit; the delivering step includes delivering the fibresuspension jet to the twin-wire nip at such a high speed to cause ayielding deflection of an outer one of the two tensioned wires whilemaintaining a substantially constant tension of the outer wire duringthe deflection by guiding the outer wire on rotating supports, at leastone of which supports is resiliently or displaceably mounted tocompensate for the deflection; wherein a speed at which the fibresuspension is delivered to the twin-wire nip is at least 300 m/min andthe tension of the outer and inner wires is at least 4 kN/m.
 2. A methodas claimed in claim 1, wherein the speed at which the fibre suspensionjet is delivered to said twin-wire nip is at least 500 m/min.
 3. Amethod as claimed in claim 1, wherein a required degree of fibreorientation in the top ply is generated in the secondary headbox.
 4. Amethod as claimed in claim 1, wherein a speed difference between thefibre suspension and the wires during the dewatering is restricted to amaximum of ±40 m/min relatively the point of minimum shear.
 5. A methodas claimed in claim 1, wherein the wire tension of at least said outerwire is at least 6 kN/m.
 6. A method as claimed in claim 1, wherein aforming roll constituting a part of said twin-wire roll nip is used forperforming said roll forming, said forming roll having a radius of atleast 0.6 m.
 7. A method for forming a multi-ply web as claimed in claim1, wherein the roll forming of the kind claimed in claim 1 is performedfor forming all plies including the primary ply.
 8. A multi-ply paper orboard product, wherein said product is formed with the method as claimedin claim
 1. 9. A method as claimed in claim 2, wherein the speed atwhich the fibre suspension jet is delivered to said twin-nip wire is atleast 800 m/min.
 10. A method as claimed in claim 5, wherein the wiretension of at least said outer wire is at least 8 kN/m.
 11. A method asclaimed in claim 6, wherein the forming roll has a radius of at least0.8 m.
 12. A forming apparatus for forming a multi-ply web of paper orboard, comprising: two tensioned wires forming a twin-wire roll nip; asecondary headbox arranged to deliver a fibre suspension jet to create atop ply onto a moist base ply on one of the two wires; and a rollforming unit for forming the top ply, the roll forming unit includes atleast one forming roll and further includes rotating supports for thewires, the roll forming unit is constructed so as to enable a yieldingdeflection of an outer of the two tensioned wires where the suspensionjet is delivered to the roll nip and such that a substantially constanttension is maintained in the outer wire during the deflection by guidingthe outer wire on the rotating supports, and at least one of therotating supports is resiliently or displaceably mounted to compensatefor the deflection.
 13. A forming apparatus as claimed in claim 12,wherein the radius of the forming roll constituting a part of saidtwin-wire nip is at least 0.6 m.
 14. A multi-ply paper or board product,wherein said product is formed with the apparatus as claimed in claim12.
 15. A forming apparatus as claimed in claim 13, wherein the radiusof the forming roll constituting a part of said twin-wire nip is atleast 0.8 m.